from collections import Counter
from itertools import groupby

N = int(input())
A = list(map(int, input().split()))

def solve(arr):
    c = Counter(arr)
    g = tuple(groupby(arr))
    if len(c) == len(g):
        return True
    return False

flag = 0

while A and A[-1] == A[0]:
    A.pop()
    flag = 1
    if not A:
        flag = 0

if solve(A):
    print(int(flag))
else:
    print(-1)